1. Field of the Invention
The invention relates to a fluidised bed furnace, in which, in a vessel having cooled or insulated walls and an inflow plate, a fluidised bed consisting of a comparatively inert material is maintained by air fed by means of air nozzles in the inflow plate, the fuel being fed to the fluidised bed after the latter has been heated up to the ignition temperature of the fuel.
2. Description of the Prior Art
Fluidised bed furnaces are used in particular for the combustion of lumpy coal, coal dust or coal slime, corresponding wastes or the like. On the one hand, the combustion air fed in via nozzles in the inflow plate leads to a uniform distribution of the air and, on the other hand, this air also effects a continuous upward and downward tubulence of the mass of the bed so that, in particular in the vertical direction, intense and thorough mixing of fuel, air and the inert material serving as the heat carrier can take place.
It is known to the state of the art to feed the fuel from above via one or more gravity shafts into the fluidised bed. This type of feed is very robust and simple, but it is no longer satisfactorily feasible in the case of relatively large fluidised beds. There should always be one feed point for each 1 to 3 m.sup.2 of fluidising plate in order to be able to charge the bed area uniformly. Although the fluidised bed, due to its character, automatically distributes the feed material, this property is restricted by inserts, such as submerged heating surfaces or the like. In any case, for a bed area of more than 3 m.sup.2, the capacity of the fluidised bed to achieve a uniform distribution is insufficient.
A pneumatic feed of the fuel via pneumatic feed devices which are provided uniformly across the inflow plate is also known to the state of the art. Pneumatic feeding, however, requires comminution and drying of the feed material since a moisture content of more than 6 to 8% in the feed material leads to sticking in the lines. Moreover, a mixture made up beforehand can comparatively readily separate again due to the different density of its constituents.
In principle, it should be stated with respect to mechanical feeding that its possible applications are restricted and, in particular, it is not possible satisfactorily to charge an inflow plate area beyond about 30 to 40 m.sup.2. An increase in the bed area would seriously impair the functioning of the fluidised bed process or would make it completely impossible.
Furthermore, in the case of pneumatic feeding, a comparatively large proportion of dust is obtained during the required comminution of the feed material, which dust proportion does not participate either in the distribution of the material in the fluidised bed or in the combustion process taking place there. This proportion is later recovered in the dust collector. Recycle is not possible since the particles cannot be held in the fluidised layer. It is therefore necessary to make provision for appropriate storage or disposal of this proportion, and this makes the corresponding type of firing substantially more expensive.